This invention generally relates to amplifiers, and more particularly to audio amplifiers and inverters for driving electric motors.
This invention overcomes the problem of amplifying signals having varying frequency in a wide range, theoretically from zero frequency to some predefined frequency, by the use of a small number of electronic components, so to provide: maximum load power, several times higher than the power which can be achieved from the same power supply by the use of the existing amplifiers of all classes without boost converter as a power supply; minimum distortion; maximum efficiency; elimination of radiated and conducted noise; maximum power supply noise rejection; efficient protection from overvoltage emerging from power supply; and minimal size of the amplifier. This technical problem is solved by new power booster amplifier, hereafter PB amplifier, according to the following specification.
Prior art discloses only amplifiers with a load at their output (FIG. 1), but does not disclose amplifiers with a load at their input (FIGS. 2 and 3) directly connected to the power supply.
Different embodiments of switching class-D amplifiers for a mono-phase load (for instance, a single voice coil loudspeaker) are disclosed in the following U.S. Pat. No. 3,585,517 issued June 1971, to R. B. Herbert (FIG. 5); U.S. Pat. No. 4,649,565 issued March 1987, to A. J. M. Kaizer et al. (FIG. 4); and Pat. No. Re. 33,333 issued Sep. 11, 1990, to W. E. Taylor, Jr. et al.
Class D amplifier for a two-phase load (for instance, a loudspeaker with two grounded voice coils) is disclosed in U.S. Pat. No. 4,360,707, issued November 1982, to J. R. Joseph et al.
Output LC filter of a class D amplifier, used for the reconstruction of the amplified signal at the output of the switching bridge, requires a large number of components of significant size, whereby the price and the dimensions of class D amplifier are considerably increased.
If the amplifier load is other than nominal for output LC filter, the amplitude response will significantly depart from the designed one. In case of a load value less than nominal, the amplitude response will be less than that designed. In case of a load value greater than nominal, the amplitude response will be greater than that designed. In case of a unloaded amplifier operating in the vicinity of parallel resonance frequency of the output LC filter, extremely high voltage is generated, which could lead to a filter capacitors breakthrough.
However, the majority of the loads used nowadays, such as electrodynamic loudspeakers, induction electric motors and brushless DC motors, are characterized by a significant inductance of their windings in relation to resistance, so that they are completely unfit for direct connection to output LC filters which are designed for a purely resistive load. One skilled in the art solves this problem by the use of a Sobel filter connected in parallel to the inductive load, whereby the total impedance becomes purely resistive at all frequencies of interest. Sobel filter consists of a resistor of the same value as the load resistance, serially connected to a specially selected capacitor canceling the effect of the load inductance. However, this solution considerably increases the dissipation of the switching bridge because additional low impedance is connected in parallel to the amplifier output.
Output LC filters of audio amplifiers and fast reacting electric motor drives feature relatively low impedance of filtering inductances, leading to an increase in current ripple through all transistors in the switching bridge, and thereby to increased dissipation on them and resistances of filter inductances.
Input LC filter of a class D amplifier, which is used to reduce injection of conducted EMI noise from the switching bridge into the cable connecting the power supply, requires bulky components, whereby the price and the dimensions of class D amplifiers are increased.
The design of such input LC filter requires special attention due to mutual interaction between its output impedance and the input impedance of the switching bridge, in order to avoid voltage oscillations at the switching bridge.
A special problem appearing during the operation of class D amplifiers with both positive and negative power supplies is the power supply bus xe2x80x9crunawayxe2x80x9d during the amplification of low frequency signals. During the positive half cycle of an input signal, the observed switch is on most of the time and power is delivered to the load and partly accumulated in the filter inductor. During the negative half cycle, the observed switch is off most of the time, and the current of the filter inductor returns back to the power supply through the diode antiparallel to the observed switch. In that case, during the positive half cycle the positive power supply voltage is decreased, while during the negative half cycle it is increased. Bearing in mind that most of power supplies are made to source, and not to sink the current, the voltage increase requires utilization of bulky capacitors or special protection circuits in the power supply.
A more detailed discussion on the problems associated with state of the art class D amplifiers is given in application notes: AN1042 xe2x80x9cHigh Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETsxe2x80x9d issued by Motorola Semiconductor in 1989, AN1013 xe2x80x9cMono Class-D Amplifiersxe2x80x9d issued by SGS-Thomson Microelectronics in 1998, AN9525 xe2x80x9cClass-D Audio II Evaluation Boardxe2x80x9d issued by Harris Semiconductor in 1996, xe2x80x9cA Real Analysis of the Power Behind Audio Power Amplifier Systemsxe2x80x9d and SLOA031 xe2x80x9cDesign Considerations for Class-D Audio Power Amplifierxe2x80x9d, both issued by Texas Instruments in 1998 and 1999, respectively.
A standard high-power amplifier consists of a switching power supply for voltage increase (boost converter) connected to a Class-D amplifier (FIG. 6).
A special kind of amplifier made of two bidirectional Cuk converters is disclosed in U.S. Pat. No. 4,186,437, issued January 1980 to S. Cuk (FIG. 7).
A switching amplifier for a two-phase load is disclosed in German patent document DE3716826A1, issued December 1988 to K. Michael et al. (FIG. 9).
Linear push-pull amplifiers for a two-phase load (for example, a loudspeaker with two grounded voice coils) are disclosed in the following U.S. Pat. No. 4,130,725, issued December 1978 to M. J. Nagel, U.S. Pat. No. 4,201,886, issued May 1980 to M. J. Nagel, and U.S. Pat. No. 4,220,832, issued September 1980 to M. J. Nagel.
Linear class AB amplifiers for a two-phase load (for example, a loudspeaker with two grounded voice coils) with variable voltage power supply are disclosed in U.S. Pat. No. 5,748,753, issued May 1998 to R. W. Carver.
The basic problem in all existing linear audio amplifiers in classes A, B and AB is the generation of heat and low efficiency during normal operation, requiring high power consumption from the power supply, which is of a specific interest for battery supplied devices such as those in cars, portable computers, radios, cassette and CD players.
The first object of the present invention is to provide a new power booster amplifier in which a two-phase load is directly connected to the power supply, on one side, and the appropriate switching half bridge, on the other side, and the switching half bridge to the bridge capacitor.
The second object of the present invention is to provide the same amplifier as the first, with the addition of an output filter between the two-phase load and the appropriate switching half bridge.